Riyad as-Salihin 15:52

'Ali (May Allah be pleased with him) reported:

The Messenger of Allah (ﷺ) said to me and to Fatimah (May Allah be pleased with her) (Ali's wife and the Prophet's daughter), "When you go to bed, recite: Takbir (Allahu Akbar) thirty-three times and Tasbih (Subhan-Allah) thirty-three times and Tahmid (Al-hamdu lillah) thirty-three times." Another narration is: The Messenger of Allah (ﷺ) said, "Recite Tasbih thirty-four times." Another narration is: The Messenger of Allah(ﷺ) said, "Recite Takbir thirty-four times." [Al-Bukhari and Muslim] .